1. Field of the Invention
The present invention relates to current feeding posts, also known as conductive mounting posts (hereinafter referred to as "feeding posts") used in electronic apparatus, such as communications equipment and computers, for structurally and electrically interconnecting substrates of various types in modular assemblies of such apparatus. The substrates may comprise conductive plates, such as bus plates and ground plates, and/or printed circuit boards on which electrical circuits are mounted having conductive terminals formed thereon for connection to power sources. More particularly, the invention relates to a flexible feeding post which provides secure mechanical and electrical interconnection of such substrates, to assure that the abutting contact surfaces of the opposite ends of each feeding post and the contact surfaces of the respective substrates interconnected thereby are in intimate and uniform engagement, to minimize the contact resistance therebetween and thus the voltage drop across the engaged contact surfaces.
2. State of the Prior Art
Modern electronic equipment comprises numerous electrical circuits of varied types which typically are mounted on several substrates, conventionally referred to as printed circuit boards. The electrical circuits are supplied with power through power distribution circuits which likewise may be formed on substrates, such as printed circuit boards, or may comprise conductive plates, known as bus plates and ground plates. FIG. 1 is a simplified, elevational view of a typical, or representative, type of such electronic equipment having modular internal organization and assembly, to which the present invention particularly relates. The modular units may be of various types, and any given module may comprise a number of printed circuit boards on each of which are mounted conventional circuits. Thus, the electronic apparatus 10 may comprise a number of subsystem modules 12, power distribution modules 14 and cooling system modules 16 and 16', the latter comprising air ducts 18 and 18' and cooling fans 20 and 20', respectively. In such a modular assembly, the substrates of each modular unit typically are arranged in parallel relationship with respect to each other, and power is supplied from appropriate power sources (not shown), to the respective circuits of the subsystem modules 12 through associated power distribution modules 14. The individual subsystem modules 12 are connected to the power distribution modules 14 by respectively associated pluralities of feeding posts 22, which are capable of carrying relatively high currents. Typically, the feeding posts 22 are integrally connected to the power distribution modules 14, in positions corresponding to the power input terminals, or contacts, of the printed circuit boards of the subsystem modules 12, such that the latter are received in abutting relationship on the corresponding contact surfaces of the feeding posts 22 and secured thereto, for example by mounting screws, so as to achieve an intimate mechanical connection therebetween having minimum electrical contact resistance and thus to avoid any undesirable voltage drop at the contacting surfaces.
As is well known, there is a stringent requirement of high packing density of the electronic elements in electronic equipment of the type with which the present invention is utilized, such as large capacity computers. The high packing density requires that a large number of circuits be mounted in close proximity on each printed circuit board. This further requires that electrical power of various different voltage levels be supplied to the circuits. Further, to assure that current is distributed uniformly to the various circuits on the printed circuit boards, it is also conventional to employ several feeding posts for distributing electrical power, even of the same voltage level, to the various circuits. As a result, it is necessary to employ a large number of feeding posts for supplying electrical power to the printed circuit boards and thus to the circuits mounted thereon. To achieve high density of the circuits mounted on a given printed circuit board, the power input terminals of each individual printed circuit board to which the feeding posts are connected preferably occupy a minimal portion of the available area of the printed circuit board, and typically are positioned in aligned relationship along the periphery of the printed circuit board. Thus, the feeding posts correspondingly must be of small dimensions and, because of the power supply requirements discussed above, must be accurately positioned to permit proper assembly of the printed circuit boards with the feeding posts.
FIG. 2 is an enlarged and exploded, perspective view of a conventional feeding post 24 which may be used to interconnect a conventional bus plate 26, which may corresond to a power distribution module 14 as seen in FIG. 1, to a corresponding circuit terminal 28 formed on a printed circuit board 30. Connecting holes 32 and 34 are formed in the bus plate 26 and the printed circuit board 30, respectively, in predetermined positions such that they are in aligned relationship when the bus plate 26 and printed circuit board 30 are assembled and interconnected by the feeding post 24. The connecting hole 34, as seen, is formed to pass centrally through the circuit terminal 28. The conventional feeding post 24 comprises a conductive, solid cylindrical stud 36 of height H, which is threaded at its opposite ends to receive corresponding screws 37, 37' each carrying a respective washer 38, 38'. For assembling the printed circuit board 30 and the base plate 26, the stud 36 is disposed therebetween with its opposite, threaded ends in alignment with the connecting holes 32 and 34, such that the screws 37, 37' may be passed through the corresponding holes 34 and 32 and be received in threaded engagement in the corresponding, internally threaded ends of the stud 36 thereby securely interconnecting the printed circuit board 30 and the base plate 26. The distance between the bus plate 1 and the printed circuit board 2 thus is defined by the height H of the cylindrical stud 36.
For the reasons described above, and particularly to satisfy the demand for high packing density of electronic components mounted on the printed circuit boards and the related power supply requirements, there has resulted a significant increase in the number of feeding posts which must be mounted on a bus plate. This contributes to a requirement for maintaining extreme accuracy both as to the height H of the cylindrical stud portion 36 of a feeding post 24, as seen in FIG. 2, and also as to precise axial alignment of the feeding post 24 and the corresponding connecting holes 32 and 34 of the bus plate 25 and the printed circuit board 30, and particularly the terminal 28 of the latter through which the connecting hole 34 passes. Absent precise alignment of these elements, mechanical distortion of the substrates and resultant stress will be produced when the substrates and associated bus plates are assembled and engaged together by the associated feeding posts.
Even if ideal conditions are satisfied in the initial mechanical assembly of the module such that no distortion and resultant stress of the substrates exist, during the subsequent operation of the electrical components, nonuniform thermal expansion of the substrates occurs which introduces mechanical distortion, or deformation, and resultant stresses. This thermal expansion, of course, is caused by the heat generated during the operation of the electrical components, e.g., heat loss from transistors, and produces nonuniform heat distribution resulting in nonuniform thermal expansion of the substrates. Further, since the substrates typically comprise a laminate of layers of different materials having different coefficients of thermal expansion, distortion of the substrates is usually unavoidable in prior art modular assemblies.
The mechanical misalignment, whether existing initially or caused by thermal effects during subsequent operation, results in a non-perpendicular mounting of the contact surfaces of the ends of the feeding posts relative to the contact surfaces of the bus plate 26 and the terminal 28 of the printed circuit board 30, such that the actual contact area of the respective, opposed contact surfaces is reduced. This effect produces an undesirable voltage drop at the interconnection, and as well can result in the generation of heat, due to the increased current density which must be carried by the reduced, common cross-sectional areas of the surfaces which are in contact. There result both adverse effects on the curcuit operation and additional heat generation, with the potential of thermal damage of the interconnecting surfaces and the portions of the substrates in their vicinity.
Thus, in the use of prior art feeding posts, it has been difficult to obtain and maintain good mechanical and electrical contact between the feeding posts and the related substrates which are interconnected thereby, and, more particularly, between the contact surfaces of the feedings posts and the intended, corresponding contact surfaces of the associated substrates, such as the terminals of circuits mounted on a printed circuit board and the surface of the bus plate. The factors contributing to that difficulty in the use of prior art feeding posts are the need to maintain precise alignment of the connecting holes of the substrates and the center line of the current feeding posts, the need to maintain uniform, consistent heights H of the cylindrical stud portion of plural such feeding posts which mechanically and electrically interconnect the associated substrates, and the problems created by thermal effects during operation. These problems are caused, to a significant extent, by the rigidity of the cylindrical stud portion of the prior art connecting posts.